In recent years there has been an increased emphasis in the use of fluid filled vibration isolators for various applications. For instance, the advent of lighter weight automobile components has generated a need for greater precision in the control of vibrations induced by normal power plant oscillations and road conditions. Because of the capability of such isolators to be designed to operate with precision, they are particularly useful in mounting engines and transmissions in modern automobiles.
The typical fluid filled vibration isolator includes a pair of opposed flexible wall chambers separated by a partition having a passage providing fluid communication between the chambers. In some fluid filled vibration isolators of this type, the passage is provided by a so-called inertia track passageway providing fluid communication in an arcuate path between the chambers. Fluid oscillates in the inertia track to provide desired dynamic stiffness characteristics at certain excitation frequencies. An example of such an isolator is disclosed in U.S. Pat. No. 4,262,886.
U.S. Pat. No. 4,159,091 discloses a fluid filled vibration isolator which utilized disc-like elements and diaphragms mounted in a passage located between opposed fluid filled chambers. The disc-like element moves in response to flexure of the chamber walls and alternating pressurization of the fluid contained therein to achieve desired dynamic stiffness characteristics.
U.S. Pat. Nos. 4,720,086 and 4,422,779 are exemplary of fluid filled vibration isolators which incorporate both an inertia track passage and a disc or diaphragm element, or decoupler, as the term is used in the art, to obtain the desired dynamic stiffness. In this type vibration isolator, the movable element which may be either a diaphragm or a disc, cooperates with the inertia track to automatically couple and decouple the inertia track relative to the pumping action from the chambers. For instance, at low amplitudes of vibration, the movable element, or decoupler, simply oscillates in response to fluid flow oscillations between the chambers, and the inertia track is relatively quiescent, or decoupled. At greater amplitudes of vibration, however, the decoupler requires all the excess fluid volume to flow through the inertia track, thereby coupling the chambers. This type vibration isolator has a relatively low dynamic stiffness at low amplitudes of vibration over a broad range of excitation frequencies and higher dynamic stiffness at higher amplitudes with regions of minimum dynamic stiffness at certain low and high excitation frequencies. Furthermore, the damping of disc or diaphragm decouplers can be characterized at a particular frequency as providing high magnitude for a narrow range of frequency for higher amplitude vibration. Reference is made to FIG. 7A.
U.S. Pat. No. 4,709,907 discloses a vibration isolator having a decoupler assembly including a mass of particulate matter loosely contained within a screened cavity in the partition. The particulate matter has a predetermined density and interstitial volume occupying less than the entire cavity when at rest therein. At low amplitudes of vibration, the particulate matter oscillates in a dispersed condition as the fluid oscillates through the screened cavity essentially bypassing the inertia track passageway between the chambers. As the amplitude of vibrations increases beyond a certain level, the particulate matter compacts alternately against the screens to throttle flow therethrough and thereby cause the fluid to oscillate in the inertia track passageway. The vibration isolator of this patent provides a quiet acting isolator. The damping of the particulate matter decoupler can be characterized when compared to the disc or diaphragm type decoupler as providing reduced magnitude of damping for a broader range of frequency at a substantially higher frequency. Reference is made to FIG. 7B which shows a phase/frequency curve for vibration isolator having a particulate decoupler.
U.K. Patent Specification No. 2,104,619A discloses a fluid filled vibration isolator utilizing a porous mass constrained between screens to throttle flow between chambers continuously. This isolator does not have an inertia track, nor does it have a decoupler. The porous mass provides a constant resistance to fluid flow.
For a more complete discussion of the structure and operational characteristics of fluid filled vibration isolators, reference is made to an article entitled A New Generation of Engine Mounts, by Marc Bernuchon, SAE Technical Paper Series 840259, 1984, the disclosure of which is incorporated by reference herein.
While vibration isolators of the type just described have certain advantages, they also have certain limitations. For example, decoupler disc are generally fabricated of rigid materials as are the cavities in which they are mounted, and such discs tend to seat abruptly under certain operating conditions. As a result, disc-type decouplers tend to generate an audible noise in operation, and this is not desirable in a commercially satisfactory fluid filled vibration isolator. Decouplers of the diaphragm type are quieter, but they have certain limitations. For instance, such decouplers are not as fatigue resistant as disc-type decouplers and do not provide complete low amplitude decoupling. Furthermore, diaphragm type decouplers do not afford the same degree of operational precision as provided by disc-type decouplers. Such isolators are suitable to provide damping of large magnitude and narrow band at a specified frequency.
Particulate matter decouplers are quiet and provide for small magnitude and broad band damping compared to the disc-type or diaphragm decouplers. However, particulate matter decouplers have certain limitations. For instance, such decouplers require loading of the particulate matter in the cage to manufacture. Also, particulate matter decouplers tend to shift the damping to higher frequencies. It is desirous to increase the magnitude of damping at selected frequency and lower the frequency at which it occurs while maintaining the broad characteristics thereof. In particular it is desirous to have a decoupler having damping characteristics of both the disc-type decoupler and the particulate type decoupler.